Tseaiment of dyed anodized aluminum



United States Patent 3,123,503 TREATMENT 0F DYED ANODIZED ALUMENUM Victor T. Humphrey's, Jersey Shore, Pa, assign'or to Koppers Company, Inc, a corporation of Delaware No Drawing. Filed Get. 4, I961, Ser. No. 142,768 3 Claims. ((31. 134-41) This invention relates to the processing of dyed anodized aluminum. In one specific aspect, it relates to a new technique for removing :from dyed anodized aluminum surfaces the deposits of scum or smut caused by the sealing operation.

A dyed anodic coating on aluminum must be sealed in order to prevent bleeding of the dyestuff through the anodic pores {and possible loss of color by sublimation of the dye. Although sealing can be accomplished by the use of boiling water alone, the majority of the useful dyes bleed extensively into the sealing bath unless hydrolyzable metallic salts are present. Conventionally, anodized aluminum that has been dyed with organic dyestuffs is sealed by immersion in a dilute aqueous solution of nickel acetate or its equivalent using a temperature at or near the boiling point of the solution. This treatment is believed to effect a rapid sealing of the anodic pores, possibly by the precipitation of an insoluble metallic hydroxide on the surface of the dyeing or :by reaction with the dyestutf to form an insoluble complex within the anodic pores.

The sealing of anodized aluminum in the manner described lhereabove was first disclosed by Tosterud in U.S. Patent 2,088,733. Tosterud found that the most useful metallic salts were the metallic acetates; e.g. the nickel, cob-alt, cadmium, zinc, barium, copper, or lead acetates. 'Iosterud also noted that the pH of an unbtutfered sealing solution of a metallic salt tends to rise during use and, accordingly, he proposed to control the pH by using buffering agents such as boric acid or various borates and acetates. The process of Tosterud is essentially that used today in commercial practice.

Unfortunately, the hydrolyzable metallic salt sealing treatment is subject to a serious disadvantage, i.e. when the anodic pores are completely sealed, a cloudy coating, probably consisting of insoluble metallic hydroxides, is deposited on the aluminum surface. In addition, on heavy anodic coatings dyed with concentrated dye solutions, a residue app-ears which is believed to be a reaction product of the dyestuif, the aluminum oxide or impurities contained therein. These coatings or residues are referred to in the art as scum or smut.

Several workers in the art have developed improvements on the basic process of Tosterud which are designed to control smut formation. For example, Glauser et al. in U.S. Patent 2,755,239 proposed a method for controlling the deposits of smut by the presence, in the sealing solution, of a water-soluble condensation product of formaldehyde and aromatic sultonic acid. Stiller, in U.S. Patent 2,888,388, proposed to control smut by the presence, [during the sealing operation, of at least (l.1% by weight of a lignosulfon-ate.

It is known that the light fastness of dyed anodized aluminum is influenced by the particular dyestuif used and also by the sealing technique. While it was recognized that the light fastness obtained using a particular color could be greatly enhanced by the use of a carefully selected sealing formulation designed for that particular color, the control of light fastness of dyed panels in this manner is commercially impractical, since it necessitates maintenance of a large number of separate sealing baths by the anodizer. I have overcome this problem by the use of my novel sea ing "formulation disclosed and claimed in my co -pending application SN. 141,021. In one embodiment my new sealing solution consists essentially of ice a minor portion of the ammonium salts or complexes of the llydrolyzable salts of nickel, cobalt and copper, the three salts being present in substantially equal parts by weight, in a major portion of water. The pH of the solution is maintained in the range of 5.5-6.0. My new sealing ]solution has the advantage of being capable of imparting improved light fastness to sealed anodized aluminum, dyed with substantially all classes of water-insoluble and solvent-soluble dyestuffs. Unfortunately, the use or this solution does not eliminate the problem of smut formation that has plagued the art for many years.

The approach to the smut problem used by Glauser and Stiller involves a {modification of the sealing solution to prevent the deposit of smut. While the suggested modifications are reasonably effective in controlling smut, they unfortunately involve a limitation on the nature of the sealing bath that loan be used. With the use of certain classes of dyestuffs to color the anodized aluminum, limiting the nature of the sealing bath incurs the disadvantage of adversely affecting the degree of light fastness that can be obtained. Furthermore, the methods of Glauser and Stiller are subject to certain objectionable side effects, such as lowering the corrosion resistance of the anodic coating, excessive bleeding out of the dyestuff and heavy streaking effects brought about by too rapid draining and drying of the anodic surface on rcmovalzfrom the hot scaling bath.

1 have discovered a unique after-treatment whichunexpectedly removes any deposits of smut, regardless of the nature of the sealing operation, and which has no harm- :ful efieot on the dyed anodized surface.

lit is, therefore, an object of the invention to provide a new after-treatment for sealed dyed anodized aluminum that completely removes smut from the surface of the aluminum without harm to the dyed anod'ic coating.

In accordance with the invention, sealed dyed anodized aluminum surfaces which are coated with smut are cleaned by intimately contacting the aluminum surface with aqueous sulfuric acid of 05-14% concentration at ambient temperatures for at least 15 seconds.

Since sulfuric acid is Widely used in the chemical removal of metal oxides from the surface of metals, i.e. pickling, it is quite surprising that this acid operates to remove the smut from the aluminum surface without damaging the sealed anodic pores. In the case of aluminum, sulfuric acid is used as the electrolyte in the anodic oxidation process. It is thus unexpected that a sulfuric acid after-treatment would not in some way etch or degrade the sealed anodic pores.

The concentration of the aqueous sulfuric acid used in the invention is quite important. If the sulfuric acid is too dilute, to if the concentration is less than about 0.5% it is ineffective in removing smut. If it is of greater than 14% concentration, the acid tends to attack the sealed anodic pores and cause visible bleeding of the color into the solution.

The sulfuric acid after-treatment is most conveniently carried out at ambient temperatures, although any temperature between about 15 and 35 C. is effective. There is no theoretical disadvantage in using temperatures lower than about 15 (1., but there is danger, if temperatures higher than 35 C. are used, that the sulfuric acid will begin to attack the sealed anodic pores, removing both the color and the anodic coating.

The time required for my novel after-treatment is not particularly critical. The sealed dyed anodized aluminum must be contacted with or immersed in a sulfuric acid solution for a time sufficient to elfect smut removal, but less than that which would cause attack on the sealed anodic pores and bleeding of the dyestuif into the solution. The time required is to some degree related to the con- 3 centration of the sulfuric acid, as is shown in the examples that; follow.

Smut can be removed from the sealed dyed anodized surface in a period as short as seconds, and in most cases a dip or contact of from 3G-l20 seconds is sulficient to effect complete removal. The sealed dyed surface can be immersed in the sulfuric acid solution for as long as 30 minutes with no apparent ill effects although, for most process work, it is obviously advantageous to effect smut removal using as short a contact time as possible.

In order to assist in rapid wetting of the smut, it is advantageous, but not essential, to add to the sulfuric acid solution a small amount, e.g. from (ll-1% by weight, of a suitable wetting agent, such as an ethylene oxide condensation product of a higher alcohol, or a condensate of formaldehyde and naphthalene sulfonic acids.

After the smut is removed by contact with the acid, the sealed dyed anodized aluminum surface is thoroughly rinsed in warm or cold water to re "love the acid. On drying the surface is clean and smut-free and requires no further buffing or polishing treatment.

My invention is further illustrated by the following examples:

Example I A panel of aluminum, which had first been anodized to give a dye receptive anodic coating of approximately 1 mil thickness, was dyed by immersion for about twenty minutes in a 0.5% by weight aqueous solution of 1,5- diamino-4,8-dihydroxy 3 sulfo-anthraquinone, a blue water soluble dye, at a temperature of about 6568 C. and a pH of 6.0, to give a deep blue coloration. The dyed anodized aluminum was then sealed by immersion for 15 to minutes in a boiling aqueous 0.5% by weight solution of nickel acetate in distilled water, followed by a further 10 minute seal in boiling distilled water. 0n removing the panel and allowing it to dry, a dull semi-opaque white film or smut was observed to be present over the entire sealed surface.

The panel was then immersed in a 7% by weight solution of sulfuric acid in water at a temperature of 24-27 C. for one minute. It was then removed, rinsed acid-free with cold water and allowed to air dry. On examination no trace of opacity or dullness could be seen.

A control panel run simultaneously, but without the sulfuric acid treatment, required several minutes hard buffing with pumice powder before complete removal of the opaque film could be effected, and the final appearance of the control panel was duller and rougher than the treated panel due to the abrasive nature of the physical treatment.

Example II A panel was dyed and scaled as in Example I above, but using copper phthalocyanine di-sulfonic acid, a blue dye suitable for anodized aluminum. Upon after-treatment with 7% by wei ht aqueous sulfuric acid as in Example I, the dyed sealed panel was clean and smutfree.

Example III A panel was dyed and sealed as in Example I above, but using Acid Black I, CI.20470, a black dye suitable for anodized aluminum. Upon after-treatment with 7% by weight aqueous sulfuric acid as in Example I, the dyed sealed panel was clean and smut-free.

Example IV A panel was dyed and sealed as in Example I above, but using Mordant Orange I, (31.14030, an orange dye suitable for anodized aluminum. Upon after-treatment with 7% by weigat aqueous sulfuric acid as in Example I, the dyed sealed panel was clean and smut-free.

Example V A panel was dyed and sealed as in Example I above,

at but using Acid Yellow 2, (11.47010, a yellow dye suitable for anodized aluminum. Upon after-treatment with 7% by weight aqueous sulfuric acid as in Example I, the dyed sealed pa el was clean and smut-free.

Example VI A panel was sealed as in Example I above, but dyed according to the procedure given in Kirby et al., US. Patent No. 2,975,081, using a 0.2% by weight solution of 1,4 dihydroxyanthraquinone in 1,2,4-trichlorobenzene. Upon after-treatment with 7% by weight aqueous sulfuric acid as in Example I, the dyed, sealed panel was clean and smut-free.

Example VII A panel was dyed and sealed as in Example VI above, but using 'l-amino-2-phenoxy-4-hydroxyanthraquinone, a red-violet solvent-soluble dye for anodized aluminum. Upon after-treatment with 7% by weight aqueous sulfuric acid as in Example I, the dyed, sealed panel was clean and smut-free.

Example VIII A panel was dyed and sealed as in Example VI above, but using 6-(4-methylanilino)-3-methyl-(3H)dibenz(f,i, j)isoquinoline-2-7-(3H)-dione, a bluish-red, solvent-soluble dye for anodized aluminum. Upon after-treatment with 7% by wegiht aqueous sulfuric acid as in Example I, the dyed, sealed panel was clean and smut-free.

Example IX Panels were prepared as in Examples I to VIII above, but sem'ed with a sealing bath containing 1% by weight of a stabilized mixture of nickel, cobalt and copper acctates, the metallic acetates being present in substantially equal proportions. The dyed anodized aluminum panels were allowed to remain in the above mentioned sealing solution for about sixty minutes, thus causing the deposition of a very heavy greyish-white smut. On treating the panels with a 7% by weight aqueous solution of sulfuric acid as described in Examples I to VIII above for about 2 minutes at 24-27" C., the smut Was completely removed, leaving the panels clean and shining, without any trace of opacity or other blemi h.

Example X A panel was prepared as in Example 1 above, but allowed to remain in the 7% sulfuric acid for one hour at a temperature of 24-27 C. No trace of color bleeding was observed, nor, after subsequent rinsing and drying, was there any visible evidence of roughening or etching of the surface. As in Example I above, the smut was completely removed.

Example XI A panel was prepared as in Example I above, but the concentration of sulfuric acid was changed from '7 to 2% by weight. It was found necessary to leave the panel immersed in the solution for from five to ten minutes to effect complete removal of the smut.

Example XII A panel was prepared as in Example I above, but the concentration of sulfuric acid was changed to 14% by weight. Very slight traces of bleeding or" the dye into the sulfuric acid solution could be observed after ten to fifteen minutes immersion but, after subsequent rinsing acid-free with water and drying, no visible etching or loss of color strength could be observed.

Example XIII When dyed anodized aluminum panels prepared as de scribed in Examples I to 1X above were treated with dilute aqueous sulfuric acid of a strength within the range of 244% by weight, but at an elevated temperature of from 37 C. up to the boiling point of the solution being used, both the color and the anodic coating were observed to be attacked and eventually destroyed. It is thus seen that the process of the invention is ineffective at high temperatures.

I claim:

1. A method of cleaning dyed anodized aluminum surfaces, which, as a result of a sealing operation using a bath containing a hydrolyzable metal salt, are coated with smut, comprising contacting said anodized aluminum surface with an aqueous sulfuric acid solution of 0.5-14% by weight concentration at ambient temperatures for a period of time at least 15 seconds and less than that which causes degrading of the sealed anodic pores resulting in observable bleeding of the dyestuff into said solution.

2. A method of cleaning dyed anodized aluminum surfaces, which, as a result of a sealing operation using a bath containing a hydrolyzable metal salt, are coated with smut, comprising immersing said anodized aluminum surface in an aqueous sulfuric acid solution of 0.544% by weight concentration at a temperature of 1535 C. for a period of time at least 15 seconds and less than that which causes degrading of the sealed anodic pores resulting in observable bleeding of the dyestuff into said solution.

3. A method of cleaning dyed anodized aluminum surfaces, which, as a result of a sealing operation using a bath containing a hydrolyzable metal salt, are coated with smut, comprising contacting said anodized aluminum surface with an aqueous sulfuric acid solution of 05-14% by weight concentration at a temperature of 15-35 C. for a period of time of at least 15 seconds and less than that which causes degrading of the sealed anodic pores resulting in observable bleeding of the dyestufi into said solution.

References Cited in the file of this patent UNITED STATES PATENTS 2,939,772 Newman et a1 June 7, 1960 3,016,293 Cybriwsky et a1 Jan. 9, 1962 FOREIGN PATENTS 548,273 Canada Nov. 5, 1957 OTHER REFERENCES Surface Treatment and Finishing of Light Metals, Metal Finishing, August 1956, pp. 5 35'8. (Copy in Div. 25.) 

1. A METHOD OF CLEANING DYED ANODIZED ALUMINUM SURFACES, WHICH, AS A RESULT OF A SEALING OPERATION USING A BATH CONTAINING A HYDROLYZABLE METAL SALT, ARE COATED WITH SMUT, COMPRISING CONTACTING SAID ANODIZED ALUMINUM SURFACE WITH AN AQUEOUS SULFURIC ACID SOLUTION OF 0.5-14% BY WEIGHT CONCENTRATION AT AMBIENT TEMPERATURES FOR A PERIOD OF TIME AT LEAST 15 SECONDS AND LESS THAN THAT WHICH CAUSES DEGRADING OF THE SEALED ANODIC PORES RESULTING IN OBSERVABLE BLEEDING OF THE DYESTUFF INTO SAID SOLUTION. 